162 



F. N. Guild — Eruptive Rocks in Mexico. 



matter. In the lapilli there are frequently embedded blocks of 

 black compact lava which have evidently rolled down from 

 above. Narrow ledges of lava are occasionally found project- 

 ing above the sand and mark the position of some outflow from 

 the crater which has solidified before reaching the base of the 

 volcano. (Fig. 2.) 



Petrography of the andesiles of Popocatepetl. — The results 

 of the petrographic study of four specimens collected from vari- 

 ous places will be given as illustrative of the variations in these 

 andesites. 



No. 1 was from one of the large blocks at the rim of the 

 crater near the Malacate. Hand specimens appear as black 



msA 



Fig. 2. Popocatepetl, showing a ridge of black andesite projecting through 

 the volcanic sand. Near the snow line. 



basaltic rocks containing numerous phenocrysts of feldspar aver- 

 aging about two millimeters in diameter, rather rounded in out- 

 line and evenly distributed. One square centimeter of surface 

 usually contains about fifteen phenocrysts. Under the micro- 

 scope the feldspar appears as broken fragments and idiomorphic 

 crystals presenting the ordinary characteristics of rocks of this 

 type ; zonal structure, abundant inclusions of the dark ground- 

 mass, and twinning according to the albite and pericline laws. 

 Optical determinations place some of the crystals in the ande- 

 sine-oligoclase, others in the andesine-labradorite series. The 

 chemical analysis given below shows that the feldspars, taken 

 as a whole, are of rather more acid type than the phenocrysts, 



